Relative utilization of isoacceptor aa-tRNAs in protein synthesis have demonstrated variations in the incorporation of the corresponding amino acid into specific sites in protein even though the isoacceptors have the same codon recognition properties. Three minor Ser-tRNAs, two of which recognize specificially the nonsense codon UGA, have been further examined. The levels of isoacceptors which recognize specific codons have been determined in human and rabbit reticulocytes and a significant correlation exists between these levels and the corresponding codewords in globin mRNA from the respective tissues. An examination of the tRNA population from mammary tissues has been undertaken.